Accelerometer sensors produced in silicon micromechanics are used nowadays in many different ways. For example, they are used in motor vehicles to release air bags, for belt tighteners, in anti-lock systems, and the like.
In the case of an accelerometer sensor of this type, as described in U.S. Pat. Nos. 4,711,128 and 4,663,972, the resiliently suspended seismic mass and the plates disposed to fit into one another to form a capacitor or rather a capacitive accelerometer sensor are made of monocrystalline silicon or quartz. The structure is first produced by anisotropic etching of trenches and then dissolved from the substrate by means of isotropic undercutting. The disadvantage of this arrangement is that during the isotropic undercutting, the material is removed from both sides, essentially in a semicylindrical form, through which means only very narrow structures can be undercut, so that the width and thus the mass of the seismic mass is limited. Furthermore, in the case of a realization in monocrystalline silicon, the contacting of the required sensor supply leads is problematic.
In the case of the accelerometer sensor described in PCT Patent Publication No. WO 92/03740, which is produced using surface micromechanics means, the capacitor structures and the suspension segments, in particular, consist of polycrystalline silicon. The disadvantage of a polycrystalline material lies in the restricted activating capacity of dopants. Especially in the case of sensors in the low-acceleration range, the suspension segments for the seismic mass must have a very long and thin design. However, to achieve high conductivity, a high level of doping is required. The component of dopant material, which is introduced into the grain boundaries of the polycrystalline material and, consequently, is not electrically activated, contributes to an increased compressive stress and, given a concentration gradient, also to a voltage gradient. This can result in a warping of the suspension segments that are fixed on two sides and cause curvature of the capacitor plates.